The present invention relates to robot arms, and particularly to articulated robot arms which are used industrially in automated manufacturing operations and the like.
Articulated robot arms are used in industry to perform such operations as welding, painting, placing objects on a conveyor in a certain orientation, assembling parts, and the like, particularly on a production line where such tasks are performed repetitively on identical workpieces.
Robot arms must learn and precisely repeat well-defined, complex motions to function in an industrial environment. Each section of an articulated arm must be translated and rotated in a well-defined way, in relation to the other sections of the arm. Such precision is required to move the terminal segment, or "hand," of the arm in the directions and to the positions which are necessary to carry out the intended function. Each section of the arm requires one or more drives, and the arm requires a programmable control system to operate the drives in coordination.
In the mechanically simplest and electronically most complex articulated arms, each jointed section of the arm is articulated with respect to the base or adjacent sections by an independent drive, and each drive is independently controlled. While very versatile in terms of its range of possible movements, such an arm requires a separate, coordinated program for each drive. To carry out a seemingly simple movement of the hand, such as translation in a straight line, by articulating segments of the arm, complementary rotations of the respective sections must be independently programmed, and the respective programs must be executed synchronously.
An articulated arm having independently driven sections also requires some of the relatively heavy drives to be mounted on the arm at a distance from its base. These drives thus are cantilevered a substantial distance from the base when the arm is extended horizontally. Much of the mass of the arm is thus needed simply to support its drives. This reduces the payload the arm can handle and the precision with which the arm can be programmed to move and reach its intended positions.
An alternative to a robot arm which has fully independent drives for each section is one in which mechanical linkages, such as parallelogram or "pantograph" linkages. These linkages join two or more adjacent sections of the arm so when one section is driven the next section moves in a complementary manner. This eliminates the need for a separate drive and separate programming for the next section. Pantographic robot arms commonly use a ball circulating screw to change the shape of the pantograph.
These pantographic arms have several disadvantages. First, the robot's wrist cannot move along a straight line with only one motor drive. The orientation of the wrist is continually changing as the arm moves, which is undesirable for most robotic applications.
Second, after a period of use the ball circulating screw will lose its precision and, due to wear, clearances will develop between its components. There is no practical method to restore the necessary accuracy to a worn ball circulating screw mechanism. The only solution is to replace the entire ball circulating screw assembly.
Third, it is difficult to protect the ball circulating screw of a pantographic robot arm from dirt, grit, water, paint, and other contaminants when the robot is operating in the presence of these contaminants. Fourth, the ball circulating screws present many lubricating problems.
Another pantographic robot arm has a pantograph comprising two parallelogram linkages in series, of which the first is supported and driven from apparatus at a base and the second is supported and driven by the first. The two parallelogram linkages are joined by two meshing sector gears. The first sector gear is fixed to a link of the first parallelogram linkage and the second sector gear is fixed to a link of the second parallelogram linkage, so the respective parallelogram linkages move reciprocally and the "hand" at the end of the arm is translated by rotating one link of one parallelogram linkage about one of its axes with respect to one adjacent link.